In the laser marking technology, different materials and mediums absorb and interact with lasers of different wavelengths, in other words, for different medium to be processed, laser devices with different wavelengths must be employed. In addition, according to “Rayleigh” rules, the theoretic resolution distance of laser marking is:
d=2.44λ f/D, where:
d represents the minimum resolution distance between two points;
λ represents the wavelength of processing light beam;
f represents the focal length of optical lens assembly;
D represents an entrance pupil diameter of optical lens assembly.
Thus, high resolution dot pitch can be obtained by adopting laser beam with ultra-short wavelength. Currently, the minimum wavelength of commercial laser device λ=266 nm. Theoretically, its resolution is five times as high as that of laser device adopting wavelength of 1064 nm, which is a very attractive choice. Nonetheless, the practical materials which can be penetrated by the ultra-short laser are rare, and fused silica is the only one ideal material, however, the refractive index of the fused silica is highly low. Ratio of refractive index to Abbe number of the fused silica is 1.476/68. If the fused silica is used as material of the lens assembly, on one hand, the thickness of the lens will increase, on the other hand, since the radius of the lens r is normally proportional to refractive index, when the fused silica is used to manufacture lens assembly, the radius r will be relative small, causing the high-order aberrations to increase, thereby making the design of high-quality Fθ lens assembly more difficult.